Is installation of cameras just plug and play? I've heard that you have to mess around with baluns. Is that right? How far should the low-voltage camera lines be from medium voltage (120 and 220 volt) lines to not cause interference?
There are a lot of ways to answer this. Much is dependant on the types of cameras you are installing. Standard video cameras (which will be the cheapest) simply require a CCTV grade coaxial cable (RG59 or RG6) and an 18 Ga. 2 conductor. There is specially made cable that has both these combined together, called Siamese cable. You will be able to get approx 1000' out of RG59 and 1200-1400' out of RG6. At those extreme lengths, being anywhere closer than 2' to HV lines (especially 220V) may cause interference. Shorter lengths will not matter so much, and intermittent contact with power cords and the like should have little to no effect.
Baluns are used to convert from coaxial cable to UTP cable such as CAT5 or 6, and bring their own problems to the table. Using them allows for easier switching to an IP camera solution in the future, since there will already be Ethernet rated cable in place. NVT has many devices of this type, as well as many others. http://www.nvt.com/
IP cameras are subject to the same limitations as any ethernet device (320ft) and PoE devices change that some as well. There are ways to lengthen an ethernet run through the use of mid-range bridges. IP cameras will be the easiest to install (especially if you will be using PoE), and will offer the most future proof solution, as they will be the standard for many more years to come. They are are anywhere from 2-4x the cost of standard video cameras, and I don't mean the cheap chinese ones. Spending money on good cameras up front is a worthwhile investment. Axis cameras, while somewhat pricey, are universally accepted by most if not all NVR's, and will set up very quickly and easily. http://www.axis.com/
Also, what cameras to get? IR? Dome cameras or bullet? What mm should they be? How do you know?
This will all depend on the conditions of the site. Do you expect vandalism? Get a rugged dome. Is it an area that has low lighting at night? Get an IR-equipped camera but don't expect too much from it at any real distance. The camera type is mostly aesthetic unless vandal resistance is a big issue. Then height is your friend. Here is a site I just found that can calculate what lenses you will need based on viewing distance and field of view. http://www.cctvlenscalculator.com/
What about viewing over the Internet? Also, how easy is it to hack them? Any additional security recommended?
I do not recommend putting these devices directly on the internet, as they are proprietary (mostly Windows based) systems and as such may have holes you won't know about. Put it behind a good firewall and gateway, and then VPN into the network to connect to it. They can be configured for direct internet access in most cases though, to be accessed by their client software by internet IP. Good luck to you if you try it.